Crystal structures of calcium phosphates related to tooth, bone and other hard tissues are determined by x-ray and neutron diffraction. Principal emphasis is on crystal imperfections and crystal chemistry. The role of fluoride in the growth of tooth mineral, its reactions with tooth crystal surfaces, its effects on enamel solubility, and its roles in caries processes are investigated. Theoretical and experimental studies are made of the caries process as a permselective membrane phenomenon, and the fundamental thermodynamic-kinetic parameters describing tooth demineralization and remineralization are sought. BIBLIOGRAPHIC REFERENCES: The Crystal Structure of Ca(BF4)2. T.H. Jordan, B. Dickens L.W. Schroeder and W.E. Brown. Acta Cryst. B31, Pt.3: 669-672, Mar 1975.